Priming a fuel system is the act of forcing fuel through the system's lines to assure uniform pressure as well as to purge any air pockets that may exist in the system. A priming device can be critical to a system when components depend on a constant flow of fuel from the time of the system start-up. An example of such a system is the fuel system for a diesel powered combustion engine.
In a diesel engine it is important to the proper operation of the engine that the fuel transfer system be sealed against the intrusion of air. Air in the system can cause failure of the injection mechanism or improper operation of the fuel system.
In addition, a diesel engine start-up requires fuel to be in constant supply from the first crank of the engine. Otherwise, excessive wear will occur in the engine and starting system as a result of continued cranking of the high compression engine while waiting for the fuel to reach the engine's combustion chamber. Priming the diesel system is particularly important when the fuel lines are empty as a result of running out the fuel supply or changing the fuel filter. Priming methods for diesel engines and the like have been widely practiced and many types of priming devices are available to address problems such as those confronted by the aforementioned fuel system.
Priming devices for a fuel system generally follow one basic form, that being a pair of check valves in series in the fuel line with a fluid pump located thereinbetween. Both check valves normally permit fluid to flow only in the direction from the fluid source to the engine. In normal operation, the fuel is permitted to flow from the fuel source through the check valves and to the engine. When the system is in priming mode, the pump draws fluid through the check valve on the fuel source side of the line, the check valve closest to the fuel source permitting fluid to flow into the pump from its drawing action while the other check valve remains closed and not permitting the pump to draw fuel from the engine. When the pump forces the fluid between the two check valves, fluid is forced only in the direction of the engine, the check valve closest to the fluid source not permitting the fuel to be pumped toward the fuel source while the other check valve allows fuel to be pumped toward the engine. Basically, the pump draws fuel from the fuel source and pumps it toward the engine without permitting back flow to the fuel source or from the engine. This device, and other variations of it, successfully performs the priming function; however, they have serious drawbacks.
The main drawback in the aforementioned system is a lag in the fuel flow which occurs as a result of the fuel having to flow past the two check valves. This can cause a shortage of fuel to the system. The defect is more profound when the system is cold and the resulting thick fuel is less viscous. Consequently, a need exists for a priming system that does not cause a lag in the fuel system in normal operation.